Bovine viral diarrhea virus (BVDV) is an enveloped virus of cattle with a single-stranded RNA genome 12.5 kb in length. The naked viral RNA is infectious upon transfection into permissive cells, being translated into a single 3988 amino acid polyprotein that is co- and post-translationally processed by viral and cellular proteases. BVDV is currently classified within the Flaviviridae family, being a member of the genus Pestivirus, along with the hog cholera virus (HCV) and border disease virus (BDV).
Bovine viral diarrhea virus naturally infects cattle, ovine, swine, caprine, buffalo and a wide variety of wild ruminants, all members of the order Artiodactyla. BVDV replicates in tissue culture cells from these species and has been often found contaminating cells from other non-related species. All known bovine cell lineages and primary cultures are susceptible to BVDV infection in vitro. In contrast, only selected cell populations have been shown to be infected in vivo. The reasons for the difference between the narrow tissue tropism in vivo and the wide cell range in vitro are not understood, but constitute a fundamental issue for the understanding of viral pathogenesis. It is likely that this difference in tissue tropism is derived from differential expression of host cell molecules that modulate virus replication and spread. The identification of host cell factors modulating virus replication may lead to the understanding of the molecular basis of virus host range and tropism.
The detailed mechanism(s) of BVDV entry into susceptible cells are unknown but certain general aspects have been elucidated. Some evidence suggests that BVDV, like other members of the Flaviviridae, initiate infection by a mechanism of receptor-mediated endocytosis, followed by low pH-dependent fusion of the virus envelope with the endosomal membrane. The BVDV envelope glycoproteins EO (gp 48) and/or E2 (gp 53) are believed to participate in the initial interactions of virions with cells. The major glycoprotein E2 (gp 53) is likely to play a major role in attachment/penetration since most monoclonal antibodies directed against E2 are neutralizing. Cell surface molecule(s) interacting with viral envelope glycoproteins to bring about BVDV entry have not been identified and characterized. Monoclonal antibodies directed against bovine cell surface proteins have been shown to partially inhibit BVDV infection, but their molecular specificity remains unknown. In addition, anti-idiotypic antibodies mimicking the BVDV E2 (gp 53) have been shown to partially prevent infection with some strains of BVDV in MDBK cells (Xue & Minocha, 1993, "Identification of the cell surface receptor for bovine viral diarrhea virus by using anti-idiotypic antibodies." J. Gen. Virol. 74: 73-9). These data taken together suggest that multiple receptors or BVDV may exist and that different strains may use different receptors to initiate infection (Collett et al., 1989 "Comparisons of the pestivirus bovine viral diarrhea virus with members of the flaviviridae." J. Gen. Virol. 69:2637-43; Xue & Minocha, 1993, supra).
Hog cholera virus (HCV, also known as classical swine fever in European countries) is a member of the genus Pestivirus, within the family Flaviviridae, as described in the 5th Report of the International Commission on Taxonomy of viruses. The virus shares a number of biological properties with the other two members of the genus: BVDV of cattle and Border Disease Virus of sheep. They have similar host ranges in vivo and in vitro. They may also cause transplacental infections in heterologous hosts; e.g. BVDV can infect pigs and HCV may infect cattle. However, infection of heterologous animal hosts with a Pestivirus does not result in acute disease. The overall genetic relatedness among the Pestiviruses is well defined by the extent of nucleotide sequence homology between the genomes of the virus species in this genus: 60-70%. Some strains of HCV are highly virulent, and can cause extremely high mortality rates in susceptible swine populations. This virus was eradicated from the U.S. in the 1970's. Harkness, J. W. (1985), "Classical swine fever and its diagnosis: A current view." Vet. Rec. 288-293; Collett, M. S., Moennig, V., and Horzinek, M., "Recent advances in Pestivirus Research." (1989) J. Gen. Virol., 70:253-266.
Border disease virus of sheep is a Pestivirus responsible for the hairy-shaker syndrome in lambs, also known as Border Disease. The syndrome is a consequence of the fetal infection which causes congenital neurologic and tegument defects. The virus appears to be unable to cause severe acute disease, in contrast with the bovine and swine Pestiviruses. Moennig, V., "Pestiviruses: A Review." Vet. Microbiol. 23(1-4): 35-54. The virus is present in the U.S. but its economic importance is commensurate with that of the sheep industry. The biology of the Border Disease virus is similar to that of BVDV, sharing .about.70% nucleotide sequence homology between genomes. P. Becher, et al "Molecular Characterization of Border Disease Virus, a Pestivirus From Sheep." Virology (1994) 198:542-51.
In 1988, House, USDA, et al reported a porcine kidney cell line IB-RS-2 Clone D10 (IB-RS-2 D10) which was said to be resistant to infection by both hog cholera virus (HCV) and BVDV (House, et al, 1988, "Characteristics of the Porcine Kidney Cell Line IB-RS-2 Clone D10 (IB-RS-2 D10) Which is Free of Hog Cholera Virus" In Vitro Cell. Dev. Bio. 24:677-682). After cloning, it was tested extensively and judged to be free of HCV based upon the results of the tests available at that time. Based on these results, USDA authorized ATCC to distribute IB-RS-2 D10. However, in 1993, as part of an ongoing program to test cell lines for the presence of Bovine Viral Diarrhea Virus, ATCC tested this cell line and found it to be infected with HCV. It is believed that the parent cell line from which the D10 clone was derived was itself infected with HCV and the cloning performed by House reduced the HCV to levels that were not detectable by the tests performed at that time, but with subsequent passages, the virus multiplied to the point where it became detectable. The resistance to BVDV infection exhibited by the D10 clone is not a true resistance. Instead, the presence of hog cholera virus in the D10 cell caused the cell to exhibit interference when the cells are inoculated with cytopathic BVD virus. The cytopathic effect was delayed and the viral titer was low after infection. The viral interference phenomenon exhibited by the host cholera virus in IB-RS-2 D10 line accounts for its apparent "resistance" to BVDV infection. However, the phenomena of interference is not the same as a cell line being refractory to infection with BVDV. (See Malmquist, et al., "Interference of Bovine Viral Diarrhea Virus by Hog Cholera Virus in Swine Kidney Cell Cultures." Am. J. Vet. Res. 26(115):1316-1327 (1965).) Therefore, while efforts have been made to identify a cell line resistant to in vitro BVDV infection, these efforts have failed heretofore.